This invention relates to a capillary unit for ink jet printers, having a nozzle for spraying a jet of ink onto a record carrier, the jet of ink breaking up into a series of droplets at a droplet formation point in front of the nozzle. A charging electrode, with which the ink droplets are selectively charged for subsequent electrical deflection, is provided in the vicinity of the droplet formation point.
Continuous pumping of ink through a fine nozzle in an ink jet printer of the above-described kind results in a continuous jet of ink which, at a given distance from the nozzle orifice, divides by spontaneous droplet formation into a string or series of droplets. Droplet formation is caused by instabilities in the ink jet as ink ejects from the nozzle's orifice. However, droplets created in spontaneous droplet formation vary in size, thereby reducing the quality of the printout obtained. Thus, attempts have been made to control droplet formation, so all droplets are of the same size in a uniform series, by mechanically vibrating the nozzle at a specific frequency.
For high-quality printout, the droplet formation point must also be set correctly in relation to the charging electrode, in addition to control of droplet formation. Proper setting of the droplet formation point in relation to the charging electrode is of the greatest importance to effective charging of the droplets and to enable correct control of droplets by the subsequent deflection electrode system.
In Electrical/Electronic Power and Control, Product Engineering, Jul. 28, 1969, pp. 66-67, an ink jet printer is described with charging electrodes in the form of two vertical, parallel plates arranged on either side of the droplet formation point. Varying the charging voltage applied to the charging electrodes charges the droplets to varying degrees, so they are deflected in the desired way in a subsequent, constant, vertical deflection field, wherein vertical movements are synchronized with horizontal movements achieved by mechanical movement of the nozzle and charging electrodes so the droplets strike the record carrier in a prescribed pattern.
The present invention refers to a type of printer with the record carrier arranged on a rotating drum, the droplet-emitting nozzle being moved perpendicular to the record carrier's direction of movement. A pulsed voltage for selective charging of the droplets to be deflected by subsequent deflection electrodes is applied to the charging electrode, so charged droplets do not reach the record carrier. For this type of printer, devising the charging electrode in the form of a plate with a through hole for passage of the droplets has proved to be advantageous.
However, one disadvantage with this type of charging electrode is that the droplet formation point cannot be visually observed. This makes the setting of the droplet formation point inside the electrode more difficult, and direct visual scrutiny of droplet formation is impossible.